The autogenic bones harvested from the bones of the same person during the same operating procedures or allogenic (living tissue transferred between two genetically different individuals of the same species), cut into smaller pieces to create morselized bone for grafting procedures in repair or augmenting skeletons.
The harvested bone may include combined cortical and cancellous bone that referred to as corticocancellous bone. The bone structure of the cancellous bone may be of the honeycomb construct containing the bone marrow. The weight bearing cortical bone is the dense and hard outer layer of the bone. In grafting, the implanted bone becomes the scaffold that needed for new bone formation. Therefore, the bone milling process must preserve the bone tissue structures and properties as much as possible.
The corticocancellous bone harvested from a spine may include larger amount of cortical bone (mostly cortical); while the bone harvested from a femoral head may include larger amount of cancellous bone (mostly cancellous).
The available bone mills generally suffer from inability to morselize of both mostly-cortical or mostly-cancellous bone; and preserving the bone tissue structure properties.
The available manually operated bone mills require excessive manual milling force and may suffer from inability to produce an adequate amount of morselized corticocancellous bone (e.g., 20 to 100 cc) within the operating room time constraints. Considering the cost per minute associated with surgical operating rooms, and the supporting staff, the manually operated bone mills may lose their price advantage; and indeed may effectively be significantly expensive, instead.
Furthermore, the available powered bone mills suffer from inability to mill the corticocancellous bone, transferring heat to the milled bone particles that may cause bone necrosis, or producing cortical thin bone shavings that may not be suitable for bone grafting.
For example, in the case of bone mill functioning similar to a coffee grinder, the milled bone remain in the milling chamber until the end of the milling process. During milling, milled particles unnecessarily undergo severe impacts with each other, with the walls of the mill, and with the dull fast rotating blade, mashing the bone structure of the cancellous bone particles and perhaps imparting heat energy that may damage the milled particles. Further, bone mill of coffee grinder design often fail to complete the needed milling. It may create very fine milled cancellous bone, “the mush” and some dust-like milled cortical bone particles, and not-milled pebble-like remaining cortical bone pieces that are beyond the mill's ability to mill. Therefore, these types of powered bone mills suffer from inability to provide the proper bone particle-size distribution profile needed for proper bone grafting.
For another example, the powered bone mill functioning as a saw-crusher often produces large bone slivers that may not be suitable for utilization in bone graft. These types of bone mills may suffer from inability to provide morselized bone with a predetermined particle-size distribution.
In some other cases, such as when the bone mill functions as a powered grater, the short-height of the cutting teeth of the blade create bone shavings which may not be suitable for proper bone grafting. Increasing the height of the cutting teeth for producing thicker strips of cortical bone requires cutting forces beyond the ability of a bone mill suitable in size for operating room environment. These types of bone mills may often suffer from inability to provide morselized corticocancellous bone with the desired particle size distribution profile and properties. Further, a portion of the harvested milled bone, trapped in the plastic blade shroud, become waste. Thus, inherently this type bone mills suffer because of low yield.
Other available bone mills utilize a cut-and-trim method of milling bone These bone mills are equipped with cortical cutting blade that is capable of cutting and sizing cortical or mostly cortical bone but fails to efficiently mill cancellous bone.
Focusing on the material cutting process, the design of a conventional cutting tooth is for cutting and removing a small amount of material per tooth throughout the cutting path. Such cutting tooth comprises a cutting edge, a primary positively angled relief surface and an angled rake. The combination of the positively angled relief surface and positively angled rake surface may create forces pulling the cutting tooth into the material for continual cutting throughout the cutting path.
Utilizing the full height of a conventional cutting tooth in cutting bone may result in grating instead of morselizing bone into small particle sizes. As such, the tooth may wedge into the bone, which may require substantial cutting force to continue cutting.
Not having the needed capable bone mill for milling cortical, cancellous, and corticocancellous bone in the operating room environment, often operating room personnel manually cut harvested bone into small pieces for grafting. Hand cutting bone is time consuming and could increase probability of infecting the graft.
Accordingly, in most, if not all, cases, conventional bone mills fail to successfully produce adequate amount of morselized bone with the needed particle-size distribution profile. (e.g., see N. T. Brewster, Mechanical consideration in impaction bone grafting. THE JOUNAL OF BONE AND JOINT SURGERY. Vol. 81-B, No. 1. January 1999).